Non-linear optical material, method of manufacturing the same and optical wavelength converter

ABSTRACT

The invention relates to a non-linear optical material comprising a salt of an organic compound with an optically active organic acid, wherein said organic compound has a conjugated π electron system containing both electron donor and acceptor groups and has at least one substituent group represented by the following formulas (1), (2) and (3) which is located outside of said conjugated π electron system. The invention also concerns a method of manufacturing the optical material and an optical converter which uses the non-linear optical material. ##STR1## According to the invention, a second-order non-linear optical material, which is readily capable of crystal growth, permits a large crystal to be readily obtained, and ensures high hardness of the crystal and gives excellent processibility and non-linear optical characteristics.

This application is a division of application Ser. No. 07/829,737, filedJan. 31, 1992, now U.S. Pat. No. 5,346,653.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-linear optical materials havingsecond-order non-linear optical characteristics, a method ofmanufacturing the same and optical wavelength converters using the same.

2. Description of the Prior Art

Heretofore, KH₂ PO₄ (abbreviated as KDP), LiNbO₃ (abbreviated as LN) andlike materials have been used for laser beam wavelength converters andelectro-optic modulators using Pockels effect. Recently, organiccompounds having large optical nonlinearity as the crystals have beenfound, and study and development concerning them are being made. Inorder for a crystallized organic compound to have second-ordernon-linear optical characteristics, it is necessary that the molecularsecond-order polarizability β of the compound is relatively large andthat the crystal structure is not centrosymmetric. For destroying thecentrosymmetric structure, the following means can be used: (1) Like thecase of 2-methyl-4-nitroaniline (abbreviated MNA), substituent group(i.e., a methyl group in this case) are introduced to decrease thesymmetry of the molecules.

(2) Like the case of methyl-(2,4-dinitrophenyl)aminopropanate,N-(5-nitro-2-pyridyl)-(S)-prolinol or like compound, an asymmetriccarbon atoms is introduced to a molecule to decrease the symmetry of themolecules.

(3) To form an organic salt such as atrans-4'-dimethylamino-N-methyl-4-stilbazolium methylsulfate.

Well-known organic compounds having second-order non-linear opticalcharacteristics and available as large crystals, are organic salts ofL-arginine phosphate monohydrate (abbreviated as LAP) and so forth.

Crystal organic compounds having second order non-linear opticalcharacteristics previously included those compounds which form molecularcrystals. These compounds have molecules which are bonded together inthe crystal by van der Waals bonds or hydrogen bonds. As the bindingforce is weak, and as the molecular symmetry is low, it is difficult toobtain a large crystal of this type of compound. Even if a large crystalcan be obtained, subsequent mechanical processing (such as cutting orpolishing to obtain an optical surface) is difficult or impossiblebecause of low mechanical strength. Thus, it is impossible to obtain asurface having satisfactory optical characteristics. Problems are,therefore, presented when the crystal is processed into a device orelement.

In ionic crystal organic compounds, such as salts, strong ionic bondsare formed in the crystal compared to van der Waals bonds or hydrogenbonds. Single crystals which are relatively large and have highmechanical strength, are obtainable. However, with the well-known ioniccrystal organic compounds having second order non-linear opticalcharacteristics, e.g., a trans-4'-dimethylamino-N-methyl-4-stilbazoliummethylsulfate, the length of the conjugated π electron system is verylarge. That is, the optical absorption maximum wavelength is comparableto that of the second harmonic of Nd:YAG or semiconductor laser beams.Therefore, wavelength converted light is absorbed by the material, andthe second harmonic can not be efficiently obtained. The LAP has smalloptical nonlinearity and particularly low efficiency of low power laserbeam conversion. Therefore, it can not be used for the wavelengthconversion of semiconductors or like laser beams.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a non-linear opticalmaterial, which is readily capable of crystal growth, can be readilygrown to a large crystal, and ensures high hardness of its crystall andhas excellent processing and non-linear optical characteristics as thecrystal. Another objects are to provide a method of manufacturing theoptical materials and to provide optical wavelength converters using theoptical materials, thus solving the above problems inherent in the priorart.

It is a further object of the invention to provide a second-ordernon-linear optical material which is readily capable of crystal growth,ensures high hardness of crystal and has excellent processing andnon-linear optical characteristics as the crystal.

It is a further object of the invention to provide a second-ordernon-linear optical material which does not absorb the second harmonic ofa Nd:YAG laser beam and thus has improved wavelength conversioncharacteristics.

It is a still further object of the invention to provide a method ofmanufacturing a second-order non-linear optical material which isreadily capable of crystal growth, permits a large crystal to be readilyobtained, ensures high hardness of its crystal and has excellentprocessing and non-linear optical characteristics as the crystal.

It is a yet further object of the invention to provide an opticalwavelength converter which uses one of the above non-linear opticalmaterials as an optical wavelength conversion crystal and has excellentoptical wavelength conversion characteristics.

The present invention relates to a non-linear optical materialcomprising a salt of an organic compound with an optically activeorganic acid, wherein said organic compound has a conjugated π electronsystem containing both electron donor and acceptor groups and has atleast one of substituent group represented by the following formulas(1), (2) and (3) which is located outside of said conjugated π electronsystem. ##STR2##

The invention relates to non-linear optical materials in which theorganic compound has a conjugated π electron system containing bothelectron donor and acceptor groups and has at least one of substituentgroup represented by the formulas (1), (2) and (3) which is locatedoutside of said conjugated π electron system. Preferably, the compoundis 1-(4-nitrophenyl)piperazine or 2-(2-aminoethylamino)-5-nitropyridineor 2-(diethylamino)ethyl 4-aminobenzoate.

The invention further relates to a method of manufacturing a non-linearoptical material, comprising producing a single crystal of a salt of anorganic compound with an optically active organic acid from a solutionof a solvent containing a mixture of water and an organic solvent bytemperature reduction or solvent evaporation, wherein said organiccompound has a conjugated π electron system containing both electrondonor and acceptor groups and has at least one of substituent grouprepresented by the above mentioned formulas (1), (2) and (3) which islocated outside of said conjugated π electron system.

The invention further relates to an optical wavelength converter inwhich a wavelength conversion crystal of a non-linear optical materialaccording to the invention is inserted into the optical cavity.

BRIEF DESCRIPTION 0F THE DRAWINGS

FIG. 1 is a view showing the crystal structure of a non-linear opticalmaterial, i.e., 1-(4-nitrophenyl)piperazine in the form of a salt ofL-tartaric acid, taken in a certain dielectric principal axis. Thematerial is described in Example 1.

FIG. 2 is a chart showing X-ray diffraction pattern of a powder specimenof the non-linear optical material, i.e., 1-(4-nitrophenyl)piperazine inthe form of a salt of L-tartaric acid, which is further described inExample 1.

FIG. 3 is a chart showing X-ray diffraction pattern of a powder specimenof 1-(4-nitrophenyl)piperazine.

FIG. 4 is a chart showing X-ray diffraction pattern of a powder specimenof L-tartaric acid.

FIG. 5 is a chart showing X-ray diffraction pattern of a powder specimenof a non-linear optical material, i.e., 1-(4-nitrophenyl)piperazine inthe form of a salt of D-phenylsuccinic acid. The material is describedin Example 2.

FIG. 6 is a chart showing X-ray diffraction pattern of a powder specimenof D-phenylsuccinic acid.

FIG. 7 is a graph showing the solubility of a non-linear opticalmaterial, i.e., 1-(4-nitrophenyl) piperazine in the form of a salt ofD-phenylsuccinic acid, with respect to water/tetrahydrofuran mixtures.The solubility is described in Example 8.

FIG. 8 is a schematic view showing a cavity type optical wavelengthconverter. The converter is described in Example 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The non-linear optical material according to the invention comprises asalt of an organic compound with an optically active organic acid,wherein said organic compound has a conjugated π electron systemcontaining both electron donor and acceptor groups and has at least oneof substituent group represented by the following formulas (1), (2) and(3) which is located outside of said conjugated π electron system.##STR3##

This means that the conjugated π electron system containing bothelectron donor and acceptor groups is retained in the non-linear opticalmaterial according to the invention. Thus, the value of the molecularsecond-order polarizability β that is required for a second-ordernon-linear optical material, can be realized with the non-linear opticalmaterial according to the invention.

In many cases, it is difficult to cause the substituent groups of theformulas (1) to (3) contained in the conjugated π electron system notedabove to accept protons because it changes the stable conjugated πelectron system. Even when protons are accepted, the electronic state ofthe conjugated π electron system is usually greatly altered. In manycases, this is undesirable. For example, it reduces β which depend onthe electronic state of the conjugated π electron system or generates alight absorption peak in the visible region. According to the invention,the above problem can be avoided because the substituent group whichconcern the conversion to a salt of the organic compound having theconjugated π electron system containing both electron donor and acceptorgroups are at feast one of the substituent groups represented by theformulas (1), (2) and (3) which are located outside of said conjugated πelectron system.

The noncentrosymmetry of crystal structure which is necessary to providethe crystal with second-order non-linear optical characteristics isintroduced into the crystal of the non-linear optical material accordingto the invention by using an optically active organic acid for producinga salt from the organic compound. Usually, an organic compound producinga molecular crystal is imparted with the optical activity in a verycomplicated way. According to the non-linear optical material of theinvention, a crystal can be produced very readily because the process isbased on a typical neutralizing reaction. Moreover, the optically activeorganic acid is naturally abundant.

The non-linear optical material according to the invention is in theform of a salt having ionic bonds, which are far stronger than van derWaals bonds or hydrogen bonds. Thus, a larger crystal than the molecularcrystals formed by van der Walls bonds or hydrogen bonds can be readilyproduced without generation of crystal defects. In addition, because ofthe very strong binding force, it is possible to obtain a crystal whichhas high hardness and can withstand mechanical processing such ascutting or polishing to obtain optical surface.

Thus, according to the invention a non-linear optical material can bereadily obtained which has excellent non-linear optical characteristics,the crystal of which is free from centrosymmetric structure, and whichis readily capable of crystal growth and ensures high hardness and goodprocessibility of its crystal.

In the non-linear optical material according to the invention, theorganic compound which is comprises the conjugated π electron systemcontaining both electron donor and acceptor groups and at least one ofthe substituent groups of the formulas (1) to (3) noted above which islocated outside of said conjugated π electron system, is preferably1-(4-nitrophenyl)piperazine or 2-(2-aminoethylamino)-5-nitropyridine.These organic compounds do not absorb the second harmonic of a Nd:YAGlaser beam. Therefore, it is possible that non-linear optical materialdoes not absorb the second harmonic of the YAG laser beam, and thus moresatisfactory wavelength conversion characteristics can be obtained.

In the non-linear optical material according to the invention, theorganic compound having the conjugated π electron system containing bothelectron donor and acceptor groups and having at least one ofsubstituent group represented by the formulas (1), (2) and (3) which islocated outside of said conjugated π electron system, is preferably1-(4-nitrophenyl)piperazine while the organic acid is preferablytartaric acid or phenylsuccinic acid. The organic compound alsopreferably is 2-(2-aminoethyl)-5-nitropyridine while the organic acid isselected from the group consisting of malic acid, mandelic acid,phenylsuccinic acid and leucinic acid. The organic compound alsopreferably is 2(diethylamino)ethyl 4-aminobenzoate while the organicacid is preferably phenylsuccinic acid. The organic acids noted aboveare inexpensive and readily available. Thus, they permit the non-linearoptical material to be readily obtained. These organic acids havecomparatively low molecular weights. Therefore, the proportion of theconjugated π electron system containing both electron donor and acceptorgroups in the crystal of the non-linear optical material is increased.It is thus possible to realize the more excellent non-linear opticalcharacteristics in a second-order non-linear optical material. Sincesuch a non-linear optical material does not absorb the second harmonicof a Nd:YAG laser beam, satisfactory wavelength conversioncharacteristics with respect to the Nd:YAG laser beam can be obtained.In particular, tartarate and phenylsuccinate, the acids of which havetwo carboxyl groups, permit greater numbers of ionic bonds to be formedin a single crystal. It is thus possible to achieve a non-lineaeroptical material which is more readily capable of crystal growth and hasincreased hardness and excellent processibility of its crystal.

In a method of manufacturing the non-linear optical material accordingto the invention, a single crystal comprising the salt of an organiccompound noted above containing the substituent group noted above withan optically active organic acid, is produced from a solution of asolvent comprising a mixture of water and an organic solvent bytemperature reduction or solvent evaporation. The salt noted above isusually difficult to dissolve in water or an organic solvent, but it isreadily dissolved in a mixtures of water and organic solvent. It is thuspossible to obtain a large crystal of the non-linear optical material inthe above method.

In the above method of manufacturing the non-linear optical material, itis preferable to use 1-(4-nitrophenyl)piperazine as the organic compoundhaving a conjugated π electron system containing both electron donor andacceptor groups and having at least one of substituent group representedby the formulas (1), (2) and (3) which is located outside of saidconjugated π electron system. it is also preferable to use tartaric acidor phenylsuccinic acid as the optically active organic acid and to usetetrahydrofuran as the organic solvent. The salt of optically activetartaric acid or phenylsuccinic acid with 1-(4-nitrophenyl)piperazinehas a considerably high solubility with respect to an adequately mixedwater/tetrahydrofuran solvent. Thus, a non-linear optical materialobtained by the above method forms a larger crystal.

In the above method of manufacturing a non-linear optical materialaccording to the invention, it is preferable to use1-(4-nitrophenyl)piperazine as the organic compound having a conjugatedπ electron system containing both electron donor and acceptor groups andhaving at least one of substituent group represented by the formulas(1), (2) and (3) which is located outside of said conjugated π electronsystem. It is also preferable to use tartaric acid as the opticallyactive organic acid and to use acetonitrile as the organic solvent. Thesalt of optically active tartaric acid or phenylsuccinic acid with1-(4-nitrophenyl)piperazine has a considerably high solubility withrespect to an adequately mixed water/tetrahydrofuran solvent. Thus, anon-linear optical material which is larger crystal can be obtained bythe above method.

According to the invention, an optical wavelength converter havingexcellent wavelength conversion characteristics can be obtained by usingthe above non-linear optical materials of the invention as a wavelengthconversion crystal which is inserted into the resonator.

Examples of the conjugated π electron system in the non-linear opticalmaterial according to the invention includ benzene derivatives, pyridinederivatives, pyrimidine derivatives, pyrazine derivatives, triazinederivatives, pyrane derivatives, pyrrole derivatives, pyrrolinederivatives, pyrazole derivatives, imidazole derivatives, furanderivatives, thiophene derivatives, thiazole derivatives, naphthalenederivatives, quinoline derivatives, indole derivatives, benzimidazolederivatives, indazole derivatives, benzofuran derivatives, benzothiazolederivatives, vinyl derivatives, allyl derivatives, etc.

Examples of electron donor groups are amino groups, alkylamino groups,dialkylamino groups, arylamino groups, diarylamino groups,alkylarylamino groups, hydroxyl groups, alkoxy groups, etc.

Examples of electron acceptor groups are nitro groups, cyano groups,formyl groups, alkylcarbonyl groups, arylcarbonyl groups, carboxylgroups, etc.

The optical activity may be either levorotaroty or dextrorotatory.Examples of the optically active acid are phenylsuccinic acid, malicacid, mandelic acid, leucinic acid, lactic acid, tartaric acid, abieticacid, quinic acid, camphoric acid, camphor-10-sulfonic acid,methoxyphenylacetic acid, 2-methoxy-2-trifluoromethylphenylacetic acid,phenylpropionic acid, etc.

Among these organic acids, tartaric acid, phenylsuccinic acid, malicacid, mandelic acid and leucinic acid are preferably used because theyare comparatively low in molecular weight as well as inexpensive andreadily available.

As the organic solvent, tetrahydrofuran, 1,4-dioxane, acetonitrile etc.may be used. These solvents are capable of being mixed with water.

Examples of the invention will now be described.

EXAMPLE 1

To 650 ml of an aqueous solution containing 9.06 g of L-tartaric acid,12.44 g of 1-(4-nitrophenyl)piperazine was added. The resulting solutionwas heated to 60° C. Then, insoluble material was filtered out, and thefiltrate was cooled to obtain a precipitate. This precipitate wasrecovered and purified by repeated recrystallization in water. Sample1-A was obtained.

FIG. 2 shows an X-ray diffraction pattern using CuK.sub.α radiation of asample obtained by powdering Sample 1-A. This X-ray diffraction patternis a different from that of the powder specimen of1-(4-nitrophenyl)piperazine shown in FIG. 3 or that of the powderspecimen of L-tartaric acid shown in FIG. 4. Thus, Sample 1-A is not amere mixture of 1-(4-nitrophenyl)piperazine and L-tartaric acid.

The NMR spectrum of a DMSO-d⁶ solution of the sample exhibited signalsdue to two kind of the methylene groups (having a proton number of 4,respectively) of the 1-(4-nitrophenyl)piperazine part at 3.48 and 3.77ppm, respectively, a signal due to the C-H groups (having a protonnumber of 2) of the L-tartaric acid part at 4.53 ppm and signals due totwo kind of the C-H groups (having a proton number of 2, respectively)of the benzene ring of 1-(4-nitrophenyl)piperazine part at 7.02 and 8.11ppm, respectively. The ratio of the peak intensities of these signalswere 2:2:1:1:1. Thus, Sample 1-A contains L-tartaric acid part and1-(4-nitrophenyl)piperazine part in a molar ratio of 1:1.

Elemental analysis values were C: 47.2%, H: 5.4%, N: 11.7% and O: 35.7%.These values correspond well with the calculated values of C: 47.1%, H:5.3%, N: 11.8% and O: 35.9%. The calculated values were made by assumingthat L-tartaric acid and 1-(4-nitrophenyl)piperazine were contained in amolar ratio of 1:1.

Sample 1-A was dissolved in a mixed water/acetonitrile mixture (having avolume ratio of 1:1) to obtain about 20 cc of saturated solution. Thesolution was left at room temperature for about 7 days to slowlyevaporate the solvent. A crystal sample (i.e., Sample 1-B) having a sizeof 2 mm×1 mm×0.1 mm was obtained.

It was found using powder X-ray analysis that this crystal had the samecrystal structure as that of Sample 1-A. As a result of X-ray crystalstructure analysis, Samples 1-A and 1-B were found to have a saltstructure as shown by the chemical formula (4) given below and a crystalstructure as shown in FIG. 1. ##STR4##

FIG. 1 is a view showing the crystal structure of the salt of1-(4-nitrophenyl)piperazine with L-tartaric acid, taken in a certainelectric principal axis (no hydrogen atom being shown). Enclosed in therectangle in FIG. 1 is a unit cell.

In this crystal structure, the molecular dipole moment of p-nitroaniline(pNA) structure parts which is related to a large β value did not canceleach other but were directed as an average in the b axis direction.

The second harmonic generation (SHG) intensity of Sample 1-A and that ofurea were measured by the Kurtz' powder method (S. K. Kurtz, J. App.Phys., 39, 3798 (1968)). A Nd:YAG laser (1,064 nm) was used as the lightsource. These crystal sample were pulverized using agate mortars toobtain a sample for SHG measurement. It was found that the efficiency ofSample 1-A was 10.1 times the urea value, demonstrating by themeasurement a excellent non-linear optical characteristic.

The SHG intensity of the powder specimen 1-(4-nitrophenyl)piperazine wasalso measured and was found to be below the limit of measurement. It wasthought that the pNA structure parts were aligned centrosymmetrically inthe 1-(4-nitrophenyl)piperazine crystal.

The SHG intensity of the powder specimen of L-tartaric acid was alsomeasured and found to be below the limit of measurement.

To determine the processibility of the crystal, the Vickers hardness ofSample 1-B was measured and found to be 63. The value was far greater(i.e., 3 to 6 times) than the SHG value of a typical prior art molecularcrystal non-linear optical material (e.g., 16 ofN-(5-nitrile-2-pyridyl)-(S)-prolinol).

Similar synthesis was made regarding the optically inactive mesotartaricacid, and the SHG was evaluated. A yellow crystal was obtained, but noSHG could be obserbed.

As a comparative example, the acceptance of protons by amino groupscontained in the conjugated π electron system will now be described. Itwas intended to produce salts of tartaric acid, malic acid and lacticacid with 2-methyl-4-nitroaniline (abbreviated as MNA) having aexcellent non-linear optical characteristic as a molecular crystalorganic compound. MNA and each of the above acids were dissolved byequal mols in water at 60° C., and the solution was then left at 5° to20° C. However, no salt could be obtained. This was thought to beattributable to withdrawing of electrons of an amino group of MNA by thenitro group resulting in diminished electron density at the amino groupwhich made proton acceptance difficult.

It was also intended to produce a salt of more strongly acidic nitricacid and MNA. Excess nitric acid and MNA were added to water at roomtemperature. The solution was then left at 5° to 10° C. As a result, asubstantially colorless, satisfactorily transparent and hard plate-likecrystal was obtained. The SHG intensity of this crystal was measuredafter pulverizing the crystal in an agate motar. The SHG efficiency wasat most 1/100 of that of urea and was difficult to measure. The salt wasunstable with respect to water. By adding water to the powdery sample,the salt was decomposed to restore the initial MNA powder.

EXAMPLE 2

2.08 g of 1-(4-nitrophenyl)piperazine was dissolved in a solventcontaining a mixture of 125 ml of tetrahydrofuran and 125 ml of tolueneat room temperaturte. The resulting solution was mixed with a solutionobtained by dissolving 1.94 g of D-phenylsuccinic acid in a solventcomprising a mixture of 125 ml of tetrahydrofuran and 125 ml of toluene.As a result, a precipitate was obtained. The solution and theprecipitate were stirred for two hours. Then, the precipitate wasrecovered and twice recrystallized with water. A crystal sample having asize of 2 mm×0.2 mm×1.0 mm (Sample 2) was obtained.

The wavelength at the absorption edge of the sample was measured by apermeation process using a spectrophotometer and found to be about 514nm.

FIG. 5 shows an X-ray diffraction pattern obtained using CuK.sub.αradiation of a sample obtained by pulverizing the obtained crystal. ThisX-ray diffraction pattern is different from those of the powder specimenof 1-(4-nitrophenyl)piperazine (shown in FIG. 3) and D-phenylsuccinicacid (shown in FIG. 6). This means that the obtained crystal is not asimple mixture of 1-(4-nitrophenyl)piperazine and D-phenylsuccinic acid.

It was recognized that the NMR spectrum of a DMSO-d⁶ solution of thesample had signals due to two kind of the methylene groups of the1-(4-nitrophenyl)piperazine part (having proton number of 4,respectively) at 2.974 , and 3.476 ppm, respectively signals due to twokind of hydrogen attached to the benzene ring of the1-(4-nitrophenyl)piperazine part (having proton number of 2,respectively) at 7.031 and 8.065 ppm, respectively signals due to twokind of the methylene group in the D-phenylsuccinic acid part (havingproton number of 1, respectively) at 2.40 and 2.838 ppm, respectively asignal due to the hydrogen bonded at the asymmetric carbon atom of theD-phenylsuccinic acid part having a proton number of 1 at 3.791 ppm anda signal due to the hydrogens attached to the benzene ring in theD-phenylsuccinic acid part (having a proton number of 5) at 7.265 ppm.The ratio of the peak intensity of these signals was 4:4:2:2:1:1:1:5.Thus, it was found that the obtained crystal smaple contained1-(4-nitrophenyl)piperazine and D-phenylsuccinic acid in a molar ratioof 1:1.

A infrared spectrum of the sample, obtained by a permeation process (KBrtablet process), did not show absorption due to the imino group thatwould generally be expected to appear in the neighborhood of 3,300 cm⁻¹with 1-(4-nitrophenyl)piperazine. In addition, regarding the absorptiondue to the carboxyl group, appearing in the neighborhood of 1,700 cm⁻¹,compared to the case of D-phenylsuccinic acid, the relative absorbance(i.e., the absorbance in D-phenylsuccinic acid or D-phenylsuccinic acidpart relative to that of other absorption) was substantially reduced toone half. Thus, the obtained sample comprises organic cations producedas a result of the acceptance of protons by mono-substituted aminogroups of 1-(4-nitrophenyl)piperazine and organic anions produced as aresult of the loss of protons by carboxyl groups of D-phenylsuccinicacid.

From the above, it has been demonstrated that the obtained crystalsample is a salt containing organic cations which are produced as aresult of the acceptance of protons by mono-substituted amino groups of1-(4-nitrophenyl)piperazine, and organic anions, which are produced as aresult of the loss of protons by carboxyl groups of D-phenylsuccinicacid, in a molar ratio of 1:1.

The SHG intensity of the sample was measured in the same way as inExample 1. It was found that the SHG efficiency of the sample was 13.2times the value obtainable with urea, thus demonstrating excellentnon-linear optical characteristics. Likewise, the SHG intensity ofpowder specimen of 1-(4-nitrophenyl)piperazine and powder specimen ofD-phenylsuccinic acid were measured and found to be below the limit ofmeasurement. From the above results, it has been demonstrated that thepNA structure parts, which are aligned centrosymmetrically in the1-(4-nitrophenyl)piperazine, are aligned noncentrosymmetrically in thecrystal of the salt of 1-(4-nitrophenyl)piperazine with D-phenylsuccinicacid. The SHG intensity of the powder specimen of D-phenylsuccinic acidwas further measured and found to be below the limit of measurement.

EXAMPLE 3

1.82 g of 2-(2-aminoethylamino)-5-nitropyridine was dissolved in 180 mlof ethanol. 5 ml of an ethanol solution containing 1.32 g of L-malicacid was then added and a precipitate was produced. After stirring thesolution for two hours, the precipitate was recovered and then dried toobtain a powder crystal sample.

It was confirmed that the sample was not a mere mixture of the rawmaterials by the same X-ray analysis as in Example 1.

The SHG intensity of the sample was measured in the manner as inExample 1. It was found that the SHG efficiency of the sample was aboutthe same as the value obtainable with urea, thus demonstrating excellentnon-linear optical characteristics. The SHG intensity of powder of2-(2-aminoethylamino)-5-nitropyridine was also measured and found to bebelow the limit of measurement. The pNA structure parts, which arealigned centrosymmetrically in the 2-(2-aminoethylamino)-5-nitropyridinecrystal, are aligned noncentrosymmetrically in this powder crystalsample. The SHG intensity of the powder specimen of L-malic acid wasalso measured and found to be below the limit of measurement.

EXAMPLE 4

1.82 g of 2-(2-aminoethylamino)-5-nitropyridine was dissolved in 200 mlof 1,4-dioxane. 15 ml of a 1,4-dioxane solution containing 1.52 g ofL-mandelic acid was then added. As a result, a precipitate was produced.After stirring the mixture for two hours, the precipitate was recoveredand then dried to obtain a powder sample.

The SHG intensity of the sample was measured in the same manner as inExample 1. It was found that the SHG efficiency of the sample was aboutthe same as the value obtainable with urea, thus demonstrating excellentnon-linear optical characteristics. The pNA strucure parts, which arealigned centrosymmetrically in the2-(2-aminomethylamino)-5-nitropyridine crystal, are alignednoncentrosymmetrically in this powder sample. The SHG intensity of aL-mandelic acid crystal sample was also measured and found to be belowthe limit of measurement.

EXAMPLE 5

1.82 g of 2-(2-aminoethylamino)-5-nitopyridine was dissolved in 100 mlof tetrahydrofuran. 100 ml of a tetrafuran solution containing 1.94 gD-phenylsuccinic acid was then added. As a result, a precipitate wasobtained. After stirring the mixture for two hours, this precipitate wasrecovered and then dried to obtain a powder sample.

The SHG intensity of the sample was measured in the same manner as inExample 1. It was found that the SHG efficiency of the sample was thesame as the value obtainable with urea, thus demonstrating excellentnon-linear optical characteristics. The pNA structure parts, which arealigned centrosymmetrically in the 2-(2-aminoethylamino)-5-nitropyridinecrystal, are aligned noncentrosymmetrically in this powder sample.

EXAMPLE 6

1.82 g of 2-(2-aminoethylamino)-5-nitropyridine was dissolved in 150 mlof 1,4-dioxane. 10 ml of 1,4-dioxane solution containing 1.32 g ofL-leucinic acid was then added. As a result, a precipitate was produced.After stirring the mixture for two hours, the precipitate was recoveredand then dried to obtain a powder sample.

The SHG intensity of the sample was measured in the same manner as inExample 1. It was found that the SHG efficiency of the sample was aboutthe same as the value obtainable with urea, thus demonstrating excellentnon-linear optical characteristics. The pNA structure parts, which arealigned centrosymmetrically in the2-(2-aminoethylamino)-5-nitropyridine, are alignednoncentrosymmetrically in this powder sample.

The SHG intensity of L-leucinic acid powder was also measured and foundto be below the limit of measurement.

EXAMPLE 7

A solution obtained by dissolving 2.36 g of 2-(diethylamino)ethyl4-aminobenzoate in a mixture of 20 ml of tetrahydrofuran and 20 ml oftoluene at room temperature, was mixed with a solution obtained bydissolving 1.94 g of D-phenylsuccinic acid in a mixture of 20 ml oftetrahydrofuran and 20 ml of toluene at room temperature. As a result, aprecipitate was produced. After stirring the mixture for two hours, theprecipitate was recovered and then dried to obtain a powder sample.

The SHG intensity of the sample was measured in the same manner as inExample 1. It was found that the SHG efficiency of the sample was aboutthe same as the value obtainable with urea, thus demonstrating excellentnon-linear optical characteristics. The SHG intensity of the powder of2-(diethylamino)ethyl 4-aminobenzoate was also measured and found to bebelow the limit of measurement. Although the crystal structure of2-(diethylamino)ethyl 4-aminobenzoate crystal is a centrosymmetric, thatof this powder sample is noncentrosymmetric.

EXAMPLE 8

A saturated solution of about 20 cc containing Sample 1-A prepared as inExample 1, and a water/tetrahydrofuran mixture (having a volume ratio of3:7), was prepared and maintained at 23° C. to slowly evaporate thesolvent. In this way, a highly transparent crystal having a size of 0.2mm×1 mm×3 mm (Sample 1-C) was obtained in about three days.

A saturated solution of about 100 cc containing Sample 1-A, prepared inExample 1, and a water/tetrahydrofuran mixture (having a volume ratio of3:7) was prepared. Then Sample 1-C was mounted in the resultingsolution. The solution was left at 23° C. to slowly evaporate of thesolvent. In this way, a highly transparent crystal having a size of 5mm×30 mm×90 mm was obtained in about 50 days.

The wavelength at the absorption edge of the crystal was measured by thepermeation process using a spectrophotometer and found to be about 505nm. As a result of powder X-ray diffraction, the crystal was found tohave a same crystal structure as that of either Sample 1-A or Sample1-B.

The crystal was also irradiated with a Q switched Nd:YAG laser beam as afundamental wave. A phase-matched green light could be observed.

Using a fundamental wave peak power of 300 MW, an output light power of150 MW was obtained. The conversion efficiency was 50%.

EXAMPLE 9

The graph of FIG. 7 shows the solubility of Sample 2, prepared as inExample 2, with respect to a water/tetrahydrofuran mixture. Sample 2 wasdissolved in the water/tetrahydrofuran mixture (having a volume ratio of3:7) in which Sample 2 is soluble very well and a saturated solution ofabout 100 cc was obtained. The saturated solution was left at 23° C. toslowly evaporate the solvent. In this way, a highly transparent crystalhaving a size of 5 mm×30 mm×30 mm was obtained in about five days.

EXAMPLE 10

A saturated solution of about 100 cc of Sample 1-B, prepared as inExample 1, in water/acetonitrile mixture (having a volume ratio of 1:1)was left at 23° C. to slowly evaporate the solvent. In this way, ahighly transparent crystal having a size of 20 mm×10 mm×1 mm wasobtained in about 15 days.

EXAMPLE 11

An optical wavelength converter according to the invention will now bedescribed with reference to FIG. 8. Referring to FIG. 8, designated at 1is a semiconductor laser, at 2 is a lens system, at 3 is a Nd:YAGcrystal, at 4 is an optical wavelength conversion crystal, at 5 is amirror, at 6 is a optical resonator, and at 7 is the second harmonic.

The optical cavity was constructed between an end surface of the Nd:YAGcrystal 3 and the surface of the mirror 5 in the optical wavelengthconverter of this example. The optical wavelength conversion crystal 4comprised a cut crystal containing the salt of1-(4-nitrophenyl)piperazine with L-tartaric acid was interposed betweenthe above two surfaces. A quartz glass having antireflection coating onthe surface was included to permit light at wavelengths of 1.06 and 0.53μm to be readily transmitted. The quartz glass was attached to the cutcrystal using matching oil or resin.

A Nd:YAG laser from the Nd:YAG crystal 3 wavelength of 1.06 μm pumped bythe semiconductor laser 1 (with wavelength of 808 nm and output power of1 W) is fundamental wave.

This fundamental wave has a very strong electric field strength in theoptical cavity. Thus, it is possible to expect a high conversionefficiency.

Using a single crystal (having a thickness of 2 to 10 mm) of a saltrepresented by the aforesaid formula (4), which was provided with theabove antireflection coating on the surface, with a semiconductor laserpower of 1 W, green light of 1 mW was obtained as the second harmonicoutput.

It is thought that a further improvement of the conversion efficiency isobtainable by improving the transparency of the crystal or selecting anoptimum direction of a fundamental wave incidence on the crystal inwhich the phase matching is possible and effective non-linearcoefficient is great while the work-off angle between the propagationdirections of the fundamental wave and the second harmonic is small.

Any material capable of direct antireflection coating of the organiccrystal may be used.

While the above examples were concerned with salts containing twodifferent kinds of ions, the invention is equally applicable to a saltcontaining three or more different kinds of ions. It is also applicableto a salt crystal containing water of crystallization.

As has been described in the foregoing, according to the invention, asecond-order non-linear optical material, which is readily capable ofcrystal growth and ensures high hardness of the crystal and hasexcellent processibility and non-linear optical characteristics as thecrystal, can be readily provided.

In a preferred mode of the invention in which the organic compound is1-(4-nitrophenyl)piperazine or 2-(2-aminoethylamino)-5-nitropyridine,the non-linear optical material does not absorb the second harmonic ofthe Nd:YAG laser beam and can provide more satisfactory wavelengthconversion characteristics.

Further, the method manufacturing of a non-linear optical materialaccording to the invention permits a large crystal non-linear opticalmaterial to be obtained because the salts noted above are capable ofbeing readily dissolved in a mixed solvent comprising water and anorganic solvent.

Furthermore, according to the invention, an optical wavelength converterhaving excellent optical wavelength conversion characteristics, can beobtained by using any of the non-linear optical materials noted above asthe optical wavelength conversion crystal.

We claim:
 1. A method of manufacturing a non-linear optical material,comprising producing a single crystal of a salt formed from an organiccompound and an optically active organic acid from a solution of asolvent containing a mixture of water and an organic solvent prepared bytemperature reduction or solvent evaporation, wherein said opticallyactive organic acid is at least one selected from the group consistingof phenylsuccinic acid, malic acid, mandelic acid, leucinic acid, lacticacid, tartaric acid, abietic acid, quinic acid, camphoric acid,camphor-10-sulfonic acid, methoxyphenylacetic acid,2-methoxy-2-trifluoromethylphenylacetic acid and phenylpropionic acid,wherein said organic compound has a conjugated π electron systemcontaining both electron donor and acceptor groups and has at least onesubstitute group selected from the group consisting of --NH₂, --NH-- and--N-- which is located outside of said conjugated π electron systemwherein:said conjugated π electron system is derived from a materialselected from the group consisting of benzene derivatives, pyridinederivatives, pyrimidine derivatives, pyrazine derivatives, triazinederivatives, pyrane derivatives, pyrrole derivatives, pyrrolinederivatives, pyrazole derivatives, imidazole derivatives, furanderivatives, thiophene derivative, thiazole derivatives, naphthalenederivatives, quinoline derivatives, indole derivatives, benzimidazolederivatives, indazole derivatives, benzofuran derivatives, benzothiazolederivatives, vinyl derivatives, and allyl derivatives; said electrondonor groups are selected from the group consisting of amino groups,monosubstituted amino groups, disubstituted amino groups, hydroxyl aminogroups, disubstituted amino groups, hydroxyl groups, and alkoxy groups;said electron acceptor groups are selected from the group consisting ofnitro groups, cyano groups, and carbonyl groups.
 2. The method ofmanufacturing a non-linear optical material according to claim 1,wherein said organic compound is 1-(4-nitrophenyl)piperazine, saidorganic acid is at least one organic acid selected from the groupconsisting of tartaric acid and phenylsuccinic acid, and said organicsolvent is tetrahydrofuran.
 3. The method of manufacturing a non-linearoptical material according to claim 1, wherein said organic compound is1-(4-nitrophenyl)piperazine, said organic acid is tartaric acid, andsaid organic solvent is acetonitrile.
 4. The method of manufacturing anon-linear optical material according to claim 1, wherein said organiccompound is a 2-(diethylamino) ethyl 4-aminobenzoate and said organicacid is a phenylsuccinic acid.